This invention relates to a heterojunction bipolar transistor (HBT). This invention also relates to a method of fabricating a heterojunction bipolar transistor.
Heterojunction bipolar transistors (HBT's) are advantageous in high-speed operation. Some HBT's have a collector-up structure in which a collector is situated over a base which is situated over an emitter situated over a substrate. In general, the base-emitter parasitic capacitance of collector-up fly HBT's is smaller than that of a normal emitter-up structure, so that the collector-up HBT's are better in high-speed operation. In addition, it is generally easy to form a common emitter circuit by using a common emitter layer in an array of collector-up HBT'S. Thus, it is hopeful that collector-up HBT's can be made into a highly-integrated circuit.
As will be explained later, a prior-art collector-up HBT has some problems.